1. Technical Field
The invention generally relates to the integrated circuit fabrication and, more specifically, to bonding of substrates having metal-dielectric patterns.
2. Background Art
In the integrated circuit fabrication industry, substrate bonding is used to join various parts of an integrated circuit (IC) together. Thermal-compression bonding is one type of bonding that is advantageous because it allows the use of metals at the bonding interface rather than just dielectrics. The metallic surface serves as a good bonding medium, and also provides additional functionality such as electrical signal propagation and thermal spreading. Unfortunately, thermal compression bonding presents many challenges in order to achieve a good quality bond.
One challenge is optimizing process parameters. Process parameters may include, for example, surface preparation before bonding such as cleans and wetting treatments, process conditions during bonding such as temperature, pressure, force, time, etc., and post-bonding treatments such as thermal cycles.
Another challenge is addressing patterned metal-dielectric mating surfaces that typically include topography, which prevents a reliable bond. In particular, metal-dielectric patterning is conventionally formed using damascene processing. Damascene processing involves fabricating interconnect metal lines by forming and filling trenches in a dielectric, and attempting to planarize the surface by chemical-mechanical polishing (CMP). Bonding of patterned surfaces takes place after the CMP step. CMP, however, leaves the metal surface non-uniform, e.g., the metal is typically concave and non-planar relative to the dielectric. As a result, the topographical differences between bonding metallic surfaces presents a challenge. Conventionally, the heating process during the bonding, which causes the reflow of the metallic pattern, has been considered sufficient to cause bonding. Unfortunately, the non-uniformity and the difference in the distances between bonding metallic surfaces does not always allow for adequate bonding via reflow, resulting in low yields.